Abstract: 【Objective】Variations in μ-calpain and physico-chemical characteristics of myofibrillar proteins in relation to the water-holding capacity in postmortem porcine muscle (1-168 h) were investigated. The study was expected to provide theoretical supports to regulate the unacceptable high drip loss of muscle and meat postmortem.【Method】Samples of porcine M. longissimus dorsi was collected for μ-calpain activity by casein zymography, myofibril fragmentation index, SDS-PAGE, myofibrillar protein surface hydrophobicity and solubility, water-holding capacity analysis. And low-field nuclear magnetic resonance relaxation T₂ (LF-NMR T₂) analysis of water mobility and distribution was also carried out.【Result】During the 1 h to 24 h postmortem, μ-calpain activity increased, while no significant changes of myofibrillar proteins degradation and MFI values were observed. Significant decreases of μ-calpain activity, with the increases in MFI occurred at 24 h postmortem, which revealed the conversion of muscle into meat. Proteolysis of muscle proteins by μ-calpain, on the one hand gave a rise in protein solubility because of more low molecular proteins emerged, on the other hand led to higher surface hydrophobicity and lower solubility due to hydrophobic groups in protein exposure to its surface and protein aggregation. Both contributed to the protein hydration characteristics. Changes of protein hydration capacity affected the attributes of muscle water populations. During the 24 h to 120 h postmortem, proteolysis degradation of myofibrillar proteins by μ-calpain occurred. The improvements of protein solubility led to the decreased population of free water P₂₃ (r=-0.246, P＜0.05）and increased population of immobilized water P₂₂(r=0.286, P＜0.01). Meanwhile, the exposure of hydrophobic residues resulted in increases of surface hydrophobicity, which was accompanied with decreases of the population of free water P₂₃ (r=0.319, P＜0.01). T₂ relaxation patterns data indicated that protein-associated water was released into the intra-myofibrillar spaces, which was supported by a high correlation between bound water P₂₁ and immobilized water P₂₂ (r=-0.890, P＜0.01). Free water and bound water converted to immobilized water. Therefore, porcine muscle water-holding capacity improved during this stage. During the 120 h to 168 h postmortem, ongoing degradation of myofibrillar proteins gave a rise in MFI values. Decreases of surface hydrophobicity and solubility caused the increased population of free water, and the decreased population of immobilized water. Immobilized water converted to free water. Thereby a flow of water from intra- to extra-myofibrillar water spaces occurred, and it can be lost as drip and water-holding capacity decreased.【Conclusion】The obtained data strongly supported that the postmortem μ-calpain was essential to changes of myofibrillar proteins hydration capacity, which moreover was found to affect the muscle 3 water populations distribution and mobility. During the 24-120 h, μ-calpain contributed to proteolysis of muscle proteins, increased proteins hydration capacity. Bound water and free water converted to immobilized water and porcine water-holding capacity improved. During the 120-168 h, low molecular proteins aggregated because of ongoing degradation, which led to the decreases of proteins hydration capacity. Immobilized water converted to free water, leading to the increases of drip loss.

Key words: porcine, postmortem aging, μ-calpain, myofibrillar proteins, hydration characteristics, water-holding capacity, water mobility